Crostini WebUI Installer: handle <esc> key and focus install button by default

Bug: 929571
Test: Enable CrostiniWebUIInstaller flag, and test manually
Change-Id: I890d5154e570c0b9f3bd59173f2696cba0211ede
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874073Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Auto-Submit: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#709798}

chrome/browser/resources/chromeos/crostini_installer/app.js

@@ -87,6 +87,15 @@ Polymer({
       }),
       callbackRouter.onCanceled.addListener(() => this.closeDialog_()),
     ];
+
+    document.addEventListener('keyup', event => {
+      if (event.key == 'Escape') {
+        this.onCancelButtonClick_();
+        event.preventDefault();
+      }
+    });
+
+    this.$$('.action-button').focus();
   },
 
   /** @override */
@@ -118,6 +127,10 @@ Polymer({
       case State.ERROR:
         this.closeDialog_();
         break;
+      case State.CANCELING:
+        // Although cancel button has been disabled, we can reach here if users
+        // press <esc> key.
+        break;
       default:
         assertNotReached();
     }